Inkjet printers typically include a drop ejection device and a supply of ink for replenishing the drop ejection device. In the case of thermal inkjet printing, the drop ejection device is typically referred to as a printhead. Printing is accomplished by the selective activation of the printhead as the printhead is moved relative to a print media.
One previously used type of inkjet printer makes use of an ink reservoir that is separately replaceable from the printhead. As ink is selectively deposited on print media, the ink reservoir replenishes the printhead with ink. In this embodiment, a region within the printhead is maintained at a slight vacuum, sometimes referred to as "backpressure." Typically, this backpressure is approximately two to three inches of water below atmospheric pressure. This backpressure within the printhead tends to prevent ink from leaking or drooling from nozzles within the printhead which can reduce print quality. If this backpressure is too large, ink flow to ink ejection chambers is reduced. This is sometimes referred to as "printhead starvation" resulting in print quality degradation and possible printhead failure.
The replaceable ink reservoir can be positioned on a scanning carriage with the printhead or positioned off the scanning carriage. In the case where the ink reservoir is mounted off carriage, the ink reservoir can be continuously in fluid communication with the printhead such as connected by a flexible conduit or intermittently connected by positioning the carriage proximate a refilling station that is in fluid communication with the printhead. The printhead is selectively replenished with ink from the refilling station. Using a replaceable ink reservoir allows for the replacement of the ink reservoir separate from the printhead allowing the use of the printhead until end of printhead life thereby reducing the cost per page of printing to the consumer.
It is frequently useful for providing a pressurized supply of ink to the printhead to achieve high flow rates or greater reliability. High flow rates are sometimes required in large format printing. Large format printing often involves printing on print media on the order of 34-54 inches in width. High flow rates are required in small format printing in cases where high print speed is required.
Various schemes have been suggested for pressurizing sources of ink. U.S. Pat. No. 5,650,811 entitled "Apparatus for Providing Ink to a Printhead", issued Jul. 22, 1997, to Seccombe et al., discloses the use of a spring for urging a piston to engage a deformable bag filled with ink.
There is an ever-present need for techniques for providing a pressurized supply of ink to achieve high flow rates and high reliability. These techniques should minimize pressure variations thereby reducing the pressure range in which the pressure regulator must compensate. In addition, these techniques should be volumetrically efficient to provide a compact ink reservoir, well suited to high volume manufacturing and be relatively low cost thereby reducing the per page print costs.
These techniques should be capable of dispensing substantially all of the ink from the replaceable ink reservoir. Stranding ink in the replaceable ink reservoir tends to reduce the consumer value. In addition, stranded ink within the replaceable ink container produces an added component in the waste stream when the ink container is discarded.
Finally, these techniques for producing a pressurized supply of ink should allow for the determination of remaining ink in the ink reservoir. It is important that the remaining ink in the ink reservoir be capable of accurately being measured to provide advance notice that the ink reservoir is in need of replacement. Another important reason for determining an amount of remaining ink in the ink reservoir is to prevent operation of the printer when the ink reservoir is exhausted of ink. In the case of thermal inkjet printers, operation of the printhead without an adequate supply of ink can result in catastrophic damage to the printhead.